It is the normal amount of errors in any printing operation.
Approximately 0.5% of all bankotes on average are replacement notes. This has been determined by surveys of random populations of notes in circulation.

There are also countries which have a higher use of Banknotes per capita, particularly lower denomination notes. These tend to have a higher number of replacement notes, and in the likes of Indonesia, Thailand, Malaysia, Singapore, a more prevalent culture of paper currency collecting.